The use of electrostatic techniques to coat electrically conductive substrates, such as metal objects, is well known and successful. The coating, such as droplets of liquid paint, is electrically charged by applying a potential difference to it and is attracted to the earthed substrate.
The conventional electrostatic coating technigue described above has not been successfully applied to the coating of pharmaceutical tablet cores or other poor electrical conductors, generally those with a resistivity of more than 10.sup.10 -10.sup.15 .OMEGA.m. Proposals have been made in which tablet cores are earthed, and a powdered coating material is directed at them through a nozzle which imparts an electrical charge to the powder. The powder coating is then fused to give a uniform coat. This method has been found inefficient, since adequate earthing of the cores has not been achieved, and the charge on the powder accumulates on the surface of the cores, repelling further charged powder. Even if the cores are carried on for example an earthed conveyor belt, the poorly conducting nature of the cores allows charge to build up.
Further, the bulk of the powder (95% in the case of corona charging) is uncharged, and does not land or stay on the cores, and must either be recovered or wasted. These difficulties lead to non-uniformity in the weight and thickness of the coating applied to the cores. This is pharmaceutically unacceptable, in particular when the core coating plays a significant role in the timing of the release of the pharmaceutical into the body after ingestion.
Improvements have been proposed, for example in WO 92/14451 which proposes moistening the cores with water prior to spraying with the charged powder, to improve the earthing of the surfaces of the cores and to encourage the powder, once on the surfaces, to remain. Even with these improvements, coating remains inherently inefficient; powder is wasted and the time necessary for complete coating is too long for efficient production.